* {
    box-sizing: border-box;
  }
  
  body {
    background-color: #658cb2;
    font-family: 'Roboto', sans-serif;
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    height: 100vh;
    overflow: hidden;
    margin: 0;
  }
  
  button {
    background-color: purple;
    color: #fff;
    border: 1px purple solid;
    font-size: 14px;
    text-transform: uppercase;
    letter-spacing: 2px;
    padding: 20px 30px;
    overflow: hidden;
    margin: 10px 0;
    position: relative;
  }
  
  button:focus {
    outline: none;
  }
  
  button .circle {
    position: absolute;
    background-color: #fff;
    width: 100px;
    height: 100px;
    border-radius: 50%; /*设置元素的圆角为50%!，(MISSING)使其呈现圆形。*/
    transform: translate(-50%, -50%) scale(0);  /*大小为0*/
    animation: scale 0.5s ease-out; /*动画时长为0.5s，ease-out时间函数，动画开始时快速加速，在末尾慢慢减速*/
  }
  
  @keyframes scale {
    to {
      transform: translate(-50%, -50%) scale(2);  /*动画会在一段时间内使元素变为原来的3倍大小*/
      opacity: 0; /*不透明度设置为0*/
    }
  }